In conventional manner, devices for unstacking such articles essentially comprise a magazine for storing the articles in a stack and an unstacking head mounted at the end of the magazine. The magazine includes means for retaining the stacked articles and means for advancing articles towards the head to feed the head with articles which the head then takes one by one from the stack. The unstacking head may be constituted, for example, by a suction cup driven with a rocking motion to come into contact with the first article in the stack and to grasp it, and driven with a reciprocating translation motion to take the article it has grasped out from the magazine, whereupon it releases it, prior to returning to the stack. During this translation motion of the said unstacking head, the stack of articles in the magazine advances so as to position a new end article for taking in the place in the preceding article.
The unstacking head is adapted to the characteristics of the articles presented thereto for the purpose of removing them one by one without damaging them. Its positioning ahead of the stack and/or its rocking motion may be controlled on the basis of detecting the format and/or the presentation of each end article to be taken.
For the purpose of storing the articles and feeding the unstacking head, such magazines have one or more slats that hold the articles and advance them as they are unstacked.
Magazines having a single slat are used for articles that are highly uniform and not very thick, e.g. letters.
These magazines are less suitable when the articles are very varied, in particular with respect to size, with it being necessary, when loading such articles, for the length of the loaded stack to be limited. Regardless of the articles in question, each time a single slat magazine is reloaded, unstacking is stopped temporarily, and in general so is the entire installation.
In order to improve uninterrupted running time of installations, particularly when used for postal sorting, magazines are generally provided with a plurality of slats. The slats split up the stack of stored articles into elementary stacks, thereby enabling a larger overall stack to be obtained. They advance the elementary stacks simultaneously along the magazine while holding them apart. If so desired, they enable new elementary stacks to be loaded into empty spaces between slats, in general in an inlet end of the magazine distant from its unstacking end, thereby topping up the overall stack while unstacking is taking place.
In such multiple slat magazines, the slats are coupled to common drive means for advancing them along the magazine, pushing the articles in front of them. In the vicinity of the unstacking heads, the slats are retracted so as to avoid the head and they enable unstacking to take place continuously from one elementary stack to the next. These retracted slats are thus made inactive and they are recycled outside the magazine back to its inlet, where they become active again.
These magazines use an endless conveyor such as a chain for advancing the slats along the magazine and for recycling them. Mechanisms associated with the slats and with the conveyor ensure that the slats are put into the active position and held there while travelling along the magazine, and that they are retracted at the unstacking end.
In addition to the considerable mechanical complexity of multiple slat magazines, these magazines are designed with a given number of slats that are distributed round the closed circuit defined by the conveyor at intervals which are fixed or which are capable of being varied only slightly. It therefore remains relatively awkward to load an appropriate elementary stack of articles into an empty space between slats, particularly when the articles are highly varied.
The object of the present invention is to avoid these drawbacks, in particular when the articles are highly varied.